1. Technical Field
The present invention relates to a driving technique using a driving signal in a capacitive load such as a piezoelectric element.
2. Related Art
A technique of ejecting a fluid in a pulse to perform cutting or excision of a target is known. For example, in the medical field, as a liquid ejecting device as a surgical tool for cutting or excising a living tissue, it is known that a fluid chamber, a volume of which is changed by driving a volume changing unit, and a nozzle communicating with the fluid chamber are provided, and the fluid is converted into a pulse flow and ejected from the nozzle in a pulse at high speed by supplying the fluid to the fluid chamber and driving the volume changing unit. As an example of the volume changing unit described above, there is an actuator configured by a capacitive load such as a piezoelectric element and operated by applying a driving signal.
In addition, there are a lot of actuators configured by a capacitive load such as a piezoelectric element and operated by applying a driving signal, such as an ejection head mounted on an ink jet printer. To generate the driving signal using an analog amplification circuit, high power is consumed since there is a large electric current flow in the analog amplification circuit. As a result, not only does the power efficiency decrease, but also a circuit board becomes large, the consumed power is changed to heat so a large heat radiation plate is necessary, and the size of the board increases more and more.
A technique is proposed in which the analog driving signal is not directly amplified, a driving waveform signal that is a driving signal reference is subjected to pulse modulation and is converted into a modulation signal, the obtained modulation signal is amplified and then allowed to pass through a low pass filter, to obtain the amplified driving signal (for example, see JP-A-2007-168172). The amplification of the modulation signal may be realized only by switching the ON/OFF switch. The low pass filter may be realized using an LC circuit formed by combining a coil and a capacitor, and thus in principle power is not consumed. For this reason, according to the proposed technique, it is possible to generate the driving signal without consuming high power, and it is possible to miniaturize the circuit board.
In the proposed technique, the low pass filter is configured by the LC circuit, and thus a gain becomes a peak at a resonance frequency of the LC circuit. Generally, the output peak is suppressed by a resistance value of an electrical load or by separately inserting a damping resistor. However, in this method, power is consumed by the resistor. It is proposed that a feedback from an output stage is performed to suppress an output peak (for example, see JP-A-2009-153272). Since a phase of the signal passing through the low pass filter is delayed to the maximal 180°, the output may oscillate when the feedback is performed with the signal of the output stage as it is. The feedback is performed after performing phase lead compensation on the signal of the output stage.
A technique (see JP-A-2005-329710) of performing feedback considering connection line resistance to prevent an operation of the driving circuit from being unstable by an influence of resistance of a connection line from the low pass filter to the capacitive load when the feedback of the signal from the output stage is performed, or a technique (see JP-A-2007-190708) of switching a carrier frequency at the time of pulse modulation according to a waveform of a driving signal to suppress power consumption is proposed.
In the related art of JP-A-2007-168172, JP-A2009-153272, JP-A-2005-329710, and JP-A-2007-190708 described above, there is a problem that a ripple (carrier ripple) of the carrier frequency removed by the low pass filter may be superimposed on the driving signal. For this reason, it is difficult to appropriately drive the actuator that is the capacitive load, particularly, it is very necessary to adjust a depth and a direction of excision in the medical field, and thus it is not allowed that a small ripple (carrier ripple) of the carrier frequency is superimposed on the driving signal.